1. Field of the Invention
This invention is a method of forming optical fibers. The method includes the transfer of gases, from a gas source to an apparatus which is rotating relative to the gas source, by means of a rotating seal, thereby preventing contamination by ambient air.
2. Disclosures of Interest
Progress over the past decade in the development of fiber fabrication technology has resulted in present day commercial fabrication of optical fibers with losses less than 5 dB/km. Such fabrication technology has been the largest single contributor to the realization of commercially viable optical communication systems. Low loss fibers are generally obtained by forming fibers in which the central region, where most of the optical transmission occurs, comprises extremely pure silica based glasses. Silica has very low loss characteristics in the spectral regions of interest, i.e. from 0.7 to 2 microns, and the high purity of the silica formed in currently available technology ensures the absence of contaminants which would otherwise increase the loss characteristics of the fiber. Dopants, such as germanium and phosphorus, are used to obtain a radially graded index of refraction so as to minimize mode loss in multi-mode fibers, as well as to obtain appropriate gradation characteristics in other fibers.
In present day technology high purity silica is obtained by starting with relatively pure glass precursor vapors, such as silicon tetrachloride and germanium tetrachloride, which are then reacted with oxygen to obtain appropriate silica based glasses. Such processes are described for example in U.S. Pat. No. Re. 28,028 and U.S. Pat. No. 4,217,027. Clearly, in the prevalent technology, which depends upon the reaction of glass precursor vapors to form glasses, significant care must be taken to ensure that ambient contaminants do not enter the gas transport system, to become ultimately of the optical fiber, adding significantly to its loss characteristics. In some fiber fabrication technologies contamination from ambient gases is avoided relatively easily using commonly available technology, including noncontaminating tubing and appropriate seals which prevent introduction of ambient air. However, in other fabrication technologies, including the prevalent modified chemical vapor deposition process described in U.S. Pat. No. 4,217,027, the glass precursor gases are reacted within a rotating glass tube. Glass, which is formed as a result of ensuing chemistry, is deposited on the inside wall of the tube to ultimately yield an optical fiber preform from which the optical fiber is drawn. In such technologies the glass precursor vapors must be transported from their source to the rotating reaction chamber without permitting the introduction of ambient contaminants. Technologically the most challenging portion of such a gas delivery system involves the region which permits rotation of the reaction tube while adequately sealing the system from ambient environments.